1. Field of the Invention
The present invention relates to a light collecting device which irradiates excitation light to a fine light emitter through an optical fiber and collects light emission from the fine light emitter, and a single-photon generation device.
2. Description of the Related Art
In order to realize electronic government (e-government), or electronic business (e-business), and furthermore, the next generation of an information society, safe and reliable cipher communication is indispensable. In cipher communication, a public encryption key and a secret encryption key are adopted.
When the public encryption key is used, since a great amount of time is required for deciphering, some security is achieved just because deciphering is so time-consuming. Such security is not sufficient. Whereas, when the secret encryption key is used, since the secret key itself may have been tapped during key distribution, the security is not sufficient, either.
In order to solve this security problem, it is proposed to use single-photons to carry key information providing unconditional security in quantum cipher communications, for example, those in conformity with the B884 protocol proposed by C. H. Bennett and G. Brassard in 1984. In addition, a semiconductor quantum dot, which is able to generate single-photons one by one by optical pulse excitation, is expected to be a promising candidate of the device for generating single-photons. In recent years, in order to increase the efficiency of producing single-photons, it has been attempted to place a single quantum dot inside a fine structure, such as a microcavity, or a small mesa structure. For example, Japanese Laid Open Patent Application No. 2000-292821 discloses a technique in this field by using the microcavity, and Japanese Laid Open Patent Application No. 2006-186084 discloses a technique in this field by using the small mesa structure.
In order to improve the throughput of the quantum cipher communications, it is important to couple an optical fiber for long distance transmission with the fine structure, which is a single-photon generator, at low loss.
FIG. 1 is a block diagram illustrating a light collecting device for a fine light emitter in the related art.
As shown in FIG. 1, in the related art, a micro-photoluminescence detection device 100 is used to optically excite the above-mentioned fine structure, couple the thus obtained light to an optical fiber 113, and collect the light. In the micro-photoluminescence detection device 100, a fine structure 101 for generating single-photons is fixed on a stage 102, which is movable in three dimensions; light from a light source 103 is irradiated on the fine structure 101 via beam splitters 104a and 104b, and the fine structure 101 is observed by using a CCD camera 105 and a monitor 106. The light source 103, the beam splitters 104a and 104b, the CCD camera 105, and the monitor 106 constitute an observing optical system.
The excitation light, which is used to generate the single-photons in the fine structure 101, is provided from an excitation laser 108, passes through a light path 109, which is different from the light path formed by the observing optical system, and a dielectric mirror 110, and is directed to the fine structure 101 by an object lens 111. The excitation laser 108, the light path 109, the dielectric mirror 110, and the object lens 111 constitute an excitation laser optical system.
The light emission generated in the fine structure 101 by the excitation light is incident into the optical fiber 113 through a condensing lens 112, and propagates to a detector 114. The condensing lens 112, the optical fiber 113, and the detector 114 constitute a fiber condensing optical system.
The above micro-photoluminescence detection device 100 allows the observing optical system, the excitation laser optical system, and the fiber condensing optical system to be adjusted separately and thus can be constructed on an optical bench easily; due to this, this kind of micro-photoluminescence detection device is widely used.
For example, this technique is also described in “S. Moehl et al., Journal of Applied Physics, 2003, Vol. 93, pp. 6265 to 6272”, and “Takemoto, Japanese Journal of Applied Physics, 2004, Vol. 43, pp. L993-L995”.
In the related art, however, on the light path between the fine structure 101 and the end 113a of the optical fiber 113, since optical elements like the dielectric mirror 110 for introducing the excitation light, and the beam splitter 104b of the observing optical system are present, the length of the lens cylinder increases, coupling efficiency between the fine structure 101 and the optical fiber 113 degrades, and consequently, loss from the fine structure 101 to the detector 114 increases.
Further, since the degree of freedom of independently adjusting the observing optical system, the excitation laser optical system, and the fiber condensing optical system is too high, it is difficult to align the observing center of the CCD camera 105 and the center axis of the optical fiber 113 with high precision, and it is difficult to obtain high positioning precision required for collecting light emissions from the fine structure 101 to the optical fiber 113.